Process for electrowinning chromium



Patented Dec. 11, 1951 FFI PROCESS FOR ELECTROWINN'IN G CHROMIUM George Westby, Boulder City, Nev., assig'nor to the United States .of America as represented by the Secretary of the Interior No Drawing. Application July 23, 1949, Serial No. 106,508

3 Claims.

(Granted under the act cfMarch 3, 1883, as

amended April 30, 1928; 370 .0. G. 757) The invention described herein may be manufactured and used by or for the Government of the United States for governmental purposes Without the payment to me of any royalty thereon in accordance with the provisions of the act of April 3-0, 1928 (Ch. 460, 45 Stat. L. 467).

'This invention relates to the electrowinning of chromium metal and more particularly to improvements therein permitting a greatly enhanced current efiiciency.

.Heretofore the process for :electrodeposition of metallic chromium from acidified solutions of chromic sulfate or the like has been widely employed. One of the early disclosures of such a process appears in U. S. Patent No. 526,114, granted September 18, 11894, to Emile 'Placet and Joseph Bonnet. Since chromium provides a bright corrosion-resistant surface, it has been widely used for plating purposes, in fact, largely replacingnie'kel for this purpose. There have been many proposals for the improvement of plating deposits by the incorporation or" various addition agents into such plating baths. For example, U. 8. Patent -No. 1,606,076, issued September 14, 1926, to Eiji Suzuki discloses the incorporation of carbolic acid, boric acid or glycerine for improving the chromium deposition in electroplating. 'U. S. Patent No. 1,705,954, dated March 1-9, 1929,130 Rudolf Auerbach, discloses the addition of colloidal silicic acid and colloidal silicates to the electroplating bath. Similarly, U. S. Patent No. 1,713,514 of May '21, 192 9, to Rudolf Appel discloses the addition of an alkali metal ioda'te in an amount not exceeding 0.05 gram per liter of bath, for improving the adherence of chrome plating to the metal object to be coated. U. S. Patent No. 1,839,905, issued January 5, 1 932, in the name of Urlyn C. Tam-ton, discloses the addition of a foaming or frothing substance to the chromium plating bath, such agents being, for example, cresylic acid, xylenol, rubber latex, or glucosides such as saponin. The purpose of these additions is stated to be the reduction of spray nuisance in metal plating. Organic compounds eapable of forming complexes with the chromium ion, such as bitartrates, glucose, cane sugar and fructose, are disclosed in U. s. Patent No. 2,114,002, granted April 12, 1938, to Oskar Kramer.

The foregoing patents disclose no simple, easy or effective means for greatly increasing the current efficiency in chromium electrowinning. As heretofore practiced, chromium .electrowinning was carried out with very low current elliciencies of the order of 20 to 30%. Obviously, in chromium electroplating, where the sole object isto produce a thin, continuous, bright film of chromium on a base, current efficiency not of exceptional importance since the cost of the electricity consumed is but a minor part of the total plating costs. iiicwever, in the electrowinning of metallic chromium where the purpose is cheaply to produce large quantities of massive chromium metal at the lowest possible cost, the-consumption of electric power is a major item of expense.

I have discovered that the incorporation of minor amounts of two readily available sub stances permits the electrowinning of metallic chromium with current eificiency of 50 to "70% or higher. Accordingly, this invention has for an object the provisionof means for electrowinning chromium metal with very high current efficiency-While at the same time providing a uniform deposition of metallic chromium on the cathode capable of being deposited until a maspsive layer is formed. Other objects will appear hereinafter as the ensuing description proceeds.

In accordance with this invention, electrowinning of metallic chromium from acidic solutions of trivalent chromium salts is carried out in-the presence of a small but effective amount of iodine and a similarly small but eifective amount of mannitol in the solution surrounding the cathode. While the invention is not to be limited by any particular theory of reaction, it is thought that oneof the effects oithe iodine-is to polarize the cathode, thus diminishing the liberation of elemental or nascent hydrogen at the cathode. It is thought that the mannitol acts as an antioxidant and stabilizes chromous ion from oxidation by air. The chromous ion is divalent and is a by-product or intermediate product of the reduction of trivalent chromium to metallic chromium. In this way, less current is used to produce hydrogen and to reduce chromic to chromous ion, thus apparently causing "the increased current efficiency.

Suitable chromium electrolytes for use in acccrdance with this invention include salts .of metallic chromium with a strong acid, preferably sulfuric acid. A suitable concentration .of chromium is from 40 to grams per liter containing .also free sulfuric acid to bring the pH to about pH 1.5 to 2.5. In addition, in order to increase the conductivity of the solution, ammonium sulfate and sodium sulfate are added in an amount suffic'ient to bring the ammonium concentration as sulfate to about 10 to 20 grams per liter, and the sodium concentration assulfate to about 20 to 40 grams per liter. When .a compartment or diaphragm type .cell is employed, aqueous sulfuric acid :is generally employed as the anolyte, such acid having .a specific gravity suitable to prevent undue migration into the catholyte, preferably a specific gravity of about 1.2. A suitable current density for employment in accordance with this invention is between about 50 to 80 amperes per square foot of cathode surface, and a voltage is employed sufficient to yield the desired current density dependent upon the electrolyte employed. As is known to those skilled in the art, usually between 2 and 5 volts will be sufficient. Under these conditions, and at a preferred temperature between 25 and 40 C., a bright, firm chromium deposit is secured by the use of iodine and mannitol as taught by my invention at a current emciency between 50 and 80% or higher.

It will be seen from the foregoing that the ordinary chromium electrowinning solutions are employed under ordinary conditions known to those skilled in the art, the improvement being the addition of iodine and mannitol to the solution surrounding the cathode. During operation, in accordance with this invention, the iodine tends to migrate to the anode where it is oxidized to iodate, for example, sodium iodate. As iodate is substantially useless in enhancing current efficiency, it is preferred to withdraw anolyte continuously or intermittently in a compartment type electrowinning cell and reduce the iodate to iodine to regenerate the same. This can be accomplished by treating the sulfuric acid anolyte with a suitable reducing agent such as sulfur dioxide, cellulose pulp or even iodides, such as sodium iodide. By this reduction, elemental iodine is liberated. By extracting the thustreated anolyte with carbon tetrachloride and then extracting the carbon tetrachloride with ethyl alcohol, an alcohol solution of iodine is obtained for return to the solution surrounding the cathode. Most of the iodine upon reduction is separated in the form of crystals which can be filtered from the sulfuric acid anolyte prior to the above described extraction step and directly returned to the catholyte. Thus, by the foregoing procedure, large losses of iodine are prevented and it is available in the desired concentration to achieve its afore-described desirable effect in the process.

Suitable quantities of iodine for use in accordance with this invention include a sufficient 'quantity of iodine to maintain the solution surrounding the cathode at an iodine concentration of about 0.02 to grams per liter and preferably the concentration is maintained within the more restricted range of about 0.4 to 1 gram per liter in order to achieve maximum effect with minimum losses.

Trivalent chromium sulfate solution was electrolyzed in a compartment-type diaphragm cell employing a carbon-rod cathode and a perforated lead plate anode. The diaphragm was of porous alundum. The anolyte solution was sulfuric acid having a specific gravity of 1.2. The catholyte solution was chromic sulfate containing 45-50 'grams per liter of chromium ion, 16.2 grams per liter of ammonium ion, 20-40 grams per liter of sodium ion and traces of iron, chloride, lead. magnesium and aluminum. To this catholyte was added iodine to equal a concentration of 0.110 grams per liter and mannitol to equal 0.07 grams per liter. The pH of the catholyte solution was 2.19. Upon electrolysis at a temperature of 312 centigrade at a current density of 70 amperes per square foot, a bright, flrm, velvet-like deposit of metallic chromium was obtained at the cathode with an average current efficiency of 66.1 per cent.

Example I] The test of Example I was repeated except that the catholyte composition was adjusted to contain 50 grams per liter of chromium ion, 16 grams per liter of ammonium ion, 63 grams per liter of sodium ion, 3.8 grams per liter of iodine and 0.005 grams per liter of mannitol. Electrolysis under the conditions of Example I yielded a firm. white, adherent deposit of metallic chromium with an average current efficiency of 51.5%, varying between 41.8% and 65.5%.

It will be seen from the foregoing that the employment of small but definite amounts of free iodine and mannitol in accordance with this invention results in the production of excellent deposits of metallic chromium capable of being built up, as in electrowinning, to extreme thicknesses while at the same time providing a very high current efliciency. Although it is preferred at present to employ free iodine and mannltol together since superior results have been noted, a distinct improvement follows from the use of either separately.

Since many apparently differing embodiments of the invention will occur to one skilled in the art, various changes can be made therein without departing from the spirit and scope thereof.

What is claimed is: I

1. In a process for the electrowinning of chromium metal from acidic chromic sulfate solutions employing a diaphragm type cell and an insoluble anode, the improvement which comprises adding from 0.4 to 1.0 gram per liter of iodine and about 0.07 gram per liter of mannitol to the solution surrounding the cathode, whereby current efficiency is improved.

2. In a process for the electrowinning of chromium metal from chromic sulfate solutions containing also ammonium and sodium sulfate, and free sulfuric acid, employing a diaphragm type cell defining an anolyte and a catholyte, the improvement which comprises incorporating in the catholyte from 0.02 to 10.0 grams of free iodine and from 0.002 to 5.0 grams of mannitol per liter of electrolyte.

3. The process of claim 2 wherein a compartment type cell is employed having a diaphragm defining an anolyte compartment wherein the iodine is oxidized to iodate, and the said iodatecontaining anolyte is withdrawn, treated with a reducing agent, the then-freed iodine is removed and returned to the catholyte.

GEORGE WESTBY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 694,658 Meurant Mar. 4, 1902 931,944 Mathers Aug. 24, 1909 1,713,514 Appel May 21, 1929 

1. IN A PROCESS FOR THE ELECTROWINNING OF CHROMIUM METAL FROM ACIDIC CHROMIC SULFATE SOLUTIONS EMPLOYING A DIAPHRAGM TYPE CELL AND AN INSOLUBLE ANODE, THE IMPROVEMENT WHICH COMPRISES ADDING FROM 0.4 TO 1.0 GRAM PER LITER OF IODINE AND ABOUT 0.07 GRAM PER LITER OF MANNITOL TO THE SOLUTION SURROUNDING THE CATHODE, WHEREBY CURRENT EFFICIENCY IS IMPROVED. 